It is known that plants can be grown in mineral wool growth substrates. Such growth substrates are typically provided as a coherent plug, block, slab or mat/blanket and generally include a binder, usually an organic binder, in order to provide structural integrity to the product.
Typically, the growth process of the plant is managed in two stages: a first stage managed by a “propagator” in which the plant is grown from seed; and a second stage managed by a “grower” during which the plant is sustained and any harvest taken. For example, in the case of the tomato plant, the propagator may plant individual tomato seeds in cylindrical plugs having a thickness in the order of 25-30 mm and a radius of around 20-30 mm. After germination of the seed, the propagator places the plug within a cuboid block to allow further growth of the root system and the plant. The individual plant within the block is then nursed until a stage when it can be transferred from the propagator to the grower.
Although often only a single plant is provided in each block, it is possible for multiple plants to be provided in a single block. In some examples, a single plant in a block is split into two by splitting a stem during an early phase of growth, resulting in two plants sharing a single root system. In another alternative, multiple plants may be grafted together and grown within a single block.
The use of a separate plug and block by the propagator is not essential for all plants, but has been described, for example, in European patent application EP2111746, as providing a number of advantages. In particular, the small size of the plug allows more regular watering of the plant in the initial stage without saturating its substrate.
After they are received from the propagator, the grower places a number of blocks on a single slab of mineral wool to form a plant growth system. The slab of mineral wool is typically encased in a foil or other liquid impermeable layer except for openings on an upper surface for receiving the blocks with the plants and a drain hole provided on the bottom surface.
During subsequent growth of the plant, water and nutrients are provided using drippers which deliver a liquid containing water and nutrients to the system either directly to the blocks or to the slabs. The water and nutrients in the blocks and slabs is taken up by the roots of the plants and the plants grow accordingly. Water and nutrients which are not taken up by the plant either remain in the substrate system or are drained through the drain hole.
There is a desire to use water and nutrients as efficiently as possible during the growing process. This is both for cost and environmental reasons. In particular, the nutrients are expensive to obtain, while waste water containing such nutrients is difficult to dispose of due to environmental legislation. These pressures will increase as raw materials (particularly fertilisers such as phosphates) become increasingly scarce. The desire to avoid such waste is matched by a desire to improve plant growth conditions, and thereby to increase the yield and quality of fruit obtained from plants in this manner.
The use of mineral wool itself provides significant benefits in this regard as compared to traditional soil-based growing methods, but there is an ongoing requirement to further improve these characteristics. In particular, there is a conflicting desire to both produce more and consume less in plant growth processes. That is, a greater yield from the plants is desired while at the same time reducing the amount of water and/or nutrients that are used. In practice, existing growing methods and/or substrates provide limitations on both these aspects.
Important qualities of plant growth systems in this context include their water retention, re-saturation and water/nutrient distribution. The water retention reflects the quantity of water that can be retained by the system while the water distribution reflects the location within the slab of the water and nutrients that are present. The re-saturation refers to the tendency of newly added liquid solution to add to the water and nutrient levels of the substrate rather than replace existing solution or be spilled.
Particular considerations which affect water retention, water distribution and re-saturation include the effect of gravity, which tends to force water downwards and thus towards the drain hole, and capillary effects which can cause water to be drawn upwards. In practice, the slabs are typically provided on a slight slope, with the drain hole located at the lowest end of the bottom surface, helping to ensure that gravity forces the water towards the drain hole. In addition to gravity and capillary effects, the flow resistance of the medium should be considered, which has the effect of preventing water passing through the slab from the drippers to the drain hole. Overall, if root and plant development is to be optimised, then it is necessary to ensure that optimal conditions are found in the region of the substrate in which the roots are growing.
As would be expected, sub-optimal water retention in the substrate can lead to either a shortage or an excess of water. In the case of shortage, this leads to water being lost, and thus wasted, through the drain hole. The water distribution is also important since it is necessary for the water within the slab to reach the plant roots. For example, when a plant has recently been placed on the slab, the roots will extend slowly into the upper regions of the slab. If water fails to reach the roots, this will result in loss of growth speed and thus loss in production. In particular, in order to ensure that the plant roots in the top region of the slab are sufficiently watered, it may be necessary for the grower to provide excessive water to the slab to maintain sufficient water around the roots, leading to greater wastage through the drain hole and extra costs. Excessive water levels can also increase the risk of fungal growth on one hand or oxygen depletion on the other which may damage the plant.
An important factor in plant growth is the retention and distribution of nutrients. Although the nutrients are typically introduced with the water, they will not necessarily be distributed and retained by the slab in the same way. The nutrients typically comprise dissolved salts comprising nitrogen, phosphorus, potassium, calcium, magnesium and similar elements. The nutrients are dissolved in the water and their movement through the slab is affected by processes such as advection, dispersion and diffusion. Advection is the movement of nutrients with the water flow through the slab, dispersion is the mixing of nutrients that occurs as they travel through complex pore structures in the slab, and diffusion relates to random movement of particles within the slab and the statistical tendency this has to reduce concentration gradients.
As with the water itself, it is important that the nutrients reach the plant roots. If nutrients are poorly distributed, or are lost from the slab, then excess nutrients may be required in the slab as a whole for the plant to receive the nutrients it requires. This is, of course, a waste of nutrients.
Another consideration that plays a role in plant growth on man made substrates is the nutrient refreshment efficiency (i.e. irrigation efficiency to refresh nutrients). This relates to whether the introduction of new nutrient solution will flush out existing nutrients in the slab. In some circumstances, it may be desirable to change the nutrient concentration within the slab during the growth process. The ability to do this will depend on whether existing nutrients can effectively be replaced through the whole slab or at least the region of the slab in which root growth takes place. Moreover, in some examples a build up of nutrients if they are not replaced can reach levels which can cause dehydration or are at least non-ideal for plant growth.
In view of this, it is recognised that the amount of water and nutrients provided to a plant plays a critical role in plant growth. This choice is typically made by analysing external factors, such as hours of sunshine or temperature and inferring the likely behaviour of the system (in terms of evaporation etc.). Whilst it is possible in green houses, for example, to control factors such as radiation by using screen and temperature by using heating systems, such systems are expensive to run and it is desirable to control the amount of water and nutrients in a manner that maximises energy savings.
It is known to measure the water and/or nutrient content within a plant growth substrate. For example, international patent application WO 2010/031773 describes a water content measuring device which determines the water content of a mineral wool substrate by measurement of a capacitance. Similarly, international patent application WO 03/005807 describes a process for measuring the oxygen level in the water in a plant growth substrate. However, although such techniques can provide useful information to the grower, they do not in of themselves ensure improved water, nutrient and oxygen content and distribution within the slab.
There is a continuing requirement to improve the irrigation of plants during plant growth. Existing techniques often result in the loss and/or overfeeding of water and/or nutrients as they are unable to offer suitable control of such properties.
For example, US 2005/0240313 and EP0300536 each describe irrigation systems including an irrigation device adapted to lower or raise the water content, so that the water content can be set to a fixed level. One disadvantage of such systems is that the EC level, and therefore nutrient level, is not suitably controlled in a timely manner. Lowering or raising water content in the known devices does not change the EC level. The EC level might change, but only if nutrient solution is added to the water.
WO 2004/109238 describes an irrigation system which takes measurements of water and nutrient levels going into the system, wherein the measurements are not taken directly on the slab. In the control unit of this system, the amount of water in the system is indicated. The EC level is inferred base on an assumption made in view of the measured amount of water.